1. Field of the Invention
The present invention relates to a dielectric filter for use in a microwave band, an antenna sharing device, and a communication device.
2. Description of the Related Art
Conventionally, band-pass filters and band-block filters have been known, in which a reactance element such as a PIN diode or variable capacitance diode is connected to a coaxial dielectric resonator, whereby the resonance frequency of each filter can be shifted by voltage control of the reactance element.
FIG. 18 is a plan view showing the configuration of a conventional variable frequency band-pass filter 1. FIG. 19 is an electric circuit diagram of the band-pass filter of FIG. 18. The filter 1 comprises resonance circuits coupled in two stages, and comprises dielectric resonators 2 and 3, coupling capacitors C5, C6 and C7, polarization capacitors C1 and C2 for producing an attenuation pole, frequency shifting capacitors C3 and C4, PIN diode D1 and D2 as reactance elements, inductors L1 and L2 to function as choke coils, control voltage supply resistors R1 and R2, capacitors C8 and C9, and a circuit substrate 5 (FIG. 18) for mounting these parts. Moreover, an input terminal electrode P1, an output terminal electrode P2, voltage control terminal electrodes CONT1 and CONT2, and ground patterns G1 and G2 are shown in FIG. 18.
Although the circuit of FIGS. 18-19 functions well, the number of parts contained in the conventional variable frequency band-pass filter 1 is large, so that miniaturization of the circuit has been difficult. A particular problem is that the space occupied by the circuit elements such as the PIN diodes or the like on the circuit substrate 5 is substantially equal to the space occupied by the dielectric resonators 2 and 3.
Moreover, conventionally, when a greater range of frequency shift is desired, the electrostatic capacitances of the frequency shifting capacitors C3 and C4 are increased. However, the interaction between the frequency shifting capacitors C3 and C4 and the PIN diodes D1 and D2 presents a problem. When the PIN diodes D1 and D2 are on, the frequency shifting capacitors C3 and C4 are dominant, respectively, as the capacitance components of the resonance circuits of the variable frequency band-pass filter 1 shown in FIG. 19. When the PIN diodes D1 and D2 are off, the capacitance between the anode and cathode terminals of each of the diodes D1 and D2 becomes dominant. For this reason, if the capacitances of the frequency shifting capacitors C3 and C4 are increased, there will be a large difference between the impedance of the resonance circuit when the PIN diodes D1 and D2 are on, and that obtained when the diodes D1 and D2 are off. Therefore, the pass-band width obtained when the PIN diodes D1 and D2 are on (that is, when the pass frequency of the filter 1 is low) is narrower than that obtained when the diodes D1 and D2 are off (that is, when the pass frequency of the filter 1 is high). Accordingly, the available range of frequency shift has a limitation. The design flexibility is low.